Curable molding compounds

Among the particulate organic fillers, wood meal is the most important material for use in thermoplastics, in particular PP. For curable molding compounds, powdered cellulose is also in this category. These finely powdered materials are actually fibrous, finely ground and specially conditioned products based on spruce or beech wood or cotton. Others include wood granulate, sawdust, shell and seed meal and, already mentioned in relation to impact modifiers, natural rubber. 

The viscosity of the curable molding compounds (before curing is completed) is usually very low (at 70-120 °C) - around 1-50 Pa s. Thermoplastic melts are between 200 Pa s and 800 Pa s (see Table 10, Sect. Processing of thermoplastic elastomers ). In contrast to injection molding of thermoplastics, duroplastics form flashes and skins the subsequent removal of which is expensive. 

Table 11 Potential resin/filler combinations for curable molding compounds...

The most important resin types used in production of curable molding compounds are phenolic, urea, melamine, unsaturated polyester, epoxide and diallyl phthalate resins. Curable molding compounds built up with these bonding agents are described in DIN 7708" (Phenoplasts and Aminoplasts), DIN 16911 and 169132 (Polyester Molding Compounds and Polyester Resin Mats), and DIN 16912 (Epoxy resin Molding Compounds). There is as yet no standard for diallyl phthalate masses, for the test method see ISO 1385 - 1.02.1977. 

The aim of standardization of certain curable molding compounds is to establish a system among the many different substances and facilitate ordering of defined materials by processing operations. The type tables define minimum specifications for typical properties determined, in accordance with relevant standards, on the basis of defined test objects. 

In parts made of curable molding compounds, deviations are influenced mainly by the following four factors [79] ... 

Whereas natural fibers, mainly in the form of short fibers and fabric chips, have foimd application above all in the curable phenoplast and aminoplast molding compounds, it was more than anything else the development of the polyesters and epoxy resins that made it necessary to develop new kinds of reinforcing fibers to optimize the properties of these products as well. Modem, high-strength, synthetic organic fibers are the results of these efforts [112]. 

Polybutadiene-type resins n. Unsaturated, thermosetting hydrocarbons cured by a peroxide-catalyzed, vinyl-type polymerization reaction, or by sodium-catalyzed polymerization of butadiene or blends of butadiene and styrene. Liquid systems, curable in the presence of monomers, are used for casting, encapsulation, and potting of electrical components, and in making laminates. Molding compounds, often containing fillers and modified with other resins or rubbers, may be compression or transfer molded. Syndiotactic 1,2-butadiene, introduced in 1974 in Japan, is thermoplastic, with semicrystalline nature, with good transparency and flexibility without plasticization. In the presence of a photosensitizer such as p,p -tetramethyl... 

Heat-curable rubber (HeR) These are typically silicon rubber compounded with catalyst, color, and additives mixed and milled into siloxane polymers. HCRs are often used in molded parts for high-temperature service or in applications that utilize their chemical and weathering resistance, but also see applications as seals and gaskets. 

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